1. Field of the Invention
The present invention belongs to a technical field of a communication navigation system for performing navigation by performing communication between a server apparatus and a communication navigation terminal apparatus and, more specifically, to a technical field of a method of retransmitting data in the case where a communication line is disconnected during data is transmission.
2. Description of the Related Art
A conventional navigation apparatus reads out map data recorded in a CD-ROM, a DVD (Digital Versatile Disk) or the like by a dedicated reading device to perform navigation for each vehicle. Thus, it is necessary to provide map data and a navigation processing device for each navigation apparatus, and there are limits in reduction of spaces and costs In addition, it is necessary to replace map data in a CD-ROM, a DVD or the like according to a place where navigation is performed. This work is troublesome. Moreover, it is necessary to purchase map data according to a place where navigation is performed, and it is also necessary for a user to purchase map data individually in order to update map data. Thus, costs for purchasing map data are increased.
Therefore, recently, there has been proposed a communication navigation terminal apparatus that is fixedly installed in a predetermined place, provided with a server apparatus having a map database and mounted on each mobile body such as a vehicle, or a communication navigation system for performing navigation of the communication navigation terminal apparatus.
This communication navigation system executes navigation by sending map data accumulated in the map database to each communication navigation terminal apparatus such as a vehicle mounted navigation apparatus, a portable navigation apparatus, a PDA (Personal Data Assistants) or a cellular phone according to a request from the communication navigation terminal apparatus via a computer network such as the Internet and a mobile body communication network.
In such a communication navigation system, it is possible that a transmission rate of map data may fall depending on a connection state of the computer network or a using state of the system. In particular, since a communication terminal apparatus is moving even during data reception, it is possible that a communication line is disconnected depending on a radio wave state of communication. Thus, if the communication line is disconnected while data is sent, the data is required to be sent from the start when the line is connected and the data is sent again.
However, since a relatively long communication time may be consumed in sending data with a large data amount such as map data, it is possible that communication costs increase if the data is sent from the start when it is sent again.